1. Field of the Invention
This invention generally relates to acoustic boundary wave devices, and more particularly, to an acoustic boundary wave device having an electrode provided on a first medium, in which a second medium is provided on the electrode and the first medium, and a third medium is provided on the second medium. Further, the present invention relates to resonators and filters using such an acoustic boundary wave device.
2. Description of the Related Art
Surface acoustic wave (SAW) devices are well known as devices using acoustic waves. The SAW devices are widely used in various circuits that process radio signals in the frequency range of 45 MHz to 2 GHz. Typically, such circuits are provided in cellular phones, and are, for example, transmit/receive filters, receive bandpass filters, local oscillation filters, antenna duplexers, IF filters and FM modulators.
In order to improve the temperature characteristic, Japanese Patent Application Publication No. 2003-209458 (Document 1) discloses a SAW device having a piezoelectric substrate on which a silicon oxide film having the sign of the temperature characteristic different from that of the piezoelectric substrate is grown. Further, the SAW device has the acoustic waves that are mostly propagated through the surface of the piezoelectric substrate. Therefore, a foreign particle on the surface of the piezoelectric substrate may vary or degrade the characteristics. For instance, the frequency may be varied or electrical loss may be increased. Thus, the SAW chip is packaged in a hermetically sealed structure. This makes it difficult to downsize the SAW device and reduce the production cost.
In order to realize improved temperature characteristics, downsizing and cost reduction, an acoustic boundary wave device is proposed in WO 98/52279 (Document 2) or Masatsune Yamaguchi, Takashi Yamashita, Ken-ya Hashimoto, Tatsuya Omori, “Highly Piezoelectric Boundary Waves in Si/SiO2/LiNbO3 Structure”, proceeding of 1998 IEEE International Frequency Control Symposium (the United States), IEEE, 1998, pp. 484-488 (Document 3). The boundary wave is propagated along the boundary between different media. FIG. 1 is a plan view of an acoustic boundary wave device disclosed in Document 2. Broken lines depict electrodes 12 provided below a third medium 16 and a second medium 14. The electrodes 12 are formed on a piezoelectric substrate 10, and the second medium 14 is provided on the electrodes 12. The second medium 14 is formed by a silicon oxide film and the third medium 16 is formed by silicon film. The electrodes 12 include a pair of comb-like electrodes 12 and a pair of reflectors 24. Extraction electrodes 22 are connected to the comb-like electrodes 20, while no extraction electrodes are connected to the reflectors 24. The second medium 14 and the third medium 16 are not formed on the extraction electrodes 22.
The duplexer or filter used close to an antenna circuit of the cellular phone is required to a power durability realizing sufficient resistance to a high input voltage. One of the ways to improve the power durability is to improve heat radiation.
FIG. 2 is a schematic cross-sectional view taken along a line A-A shown in FIG. 1. The electrodes are formed on the substrate 10, and the second medium 14 is provided so as to cover the electrode 12. Further, the third medium 16 is provided on the second medium 14. As indicated by arrows, some heat generated in the electrodes 12 conducts through the substrate 10 and is then radiated. In contrast, heat conducting to the third medium 16 through the second medium 14 is not radiated, and raises the temperature of the electrodes 12. This degrades the power durability.